1. Field of the Invention
The present invention relates to a high molecular weight electret film and to a sleeve for conveying a developing agent by providing the high molecular weight electret film on an electrically conducting sleeve.
2. Description of the Prior Art
A so-called magnetic-brush developing device has heretofore been widely used as a developing device for commercial electrophotographic apparatuses, according to which image is developed and is formed by electrically charging a two-component-type developing agent of a mixture of, for example, a magnetic carrier and an electroscopic toner or by electrically charging a one-component-type developing agent which by itself contains a magnetic powder, by forming a magnetic brush of the developing agent on a sleeve for conveying the developing agent that contains a magnet therein, conveying the magnetic brush onto the surface of a photosensitive material having electrostatic latent image, and by rubbing the surface with the magnetic brush in a bias electric field.
A method has recently been proposed to effect the developing by conveying the developing agent by using, as a sleeve for conveying the developing agent, an electrically conducting sleeve on which a high molecular weight electret film is provided. In a developing device using such a sleeve for conveying the developing agent, use is made of a nonmagnetic one-component-type developing agent which is conveyed relying only upon the electric attractive force (coulomb force) without using magnet and without forming a magnetic brush of the developing agent, giving advantage that the apparatus can be simply constructed.
Here, the electret is a dielectric that exhibits a permanent electric polarizing action. As high molecular electrets, there have been known electrets of various thermoplastic resins. Among them, an ethylene tetrafluoride/propylene hexafluoride copolymer (FEP) and a poly(4-methylpentene) have been known for their very high electric charge-retaining property.
However, the above-mentioned FEP and the like are sparingly soluble in a solvent making it difficult to prepare a suitable coating solution and, hence, making it difficult to form a film on the electrically conducting sleeve. As the electret film to be provided on the electrically conducting sleeve, therefore, the one made of a polystyrene has been proposed from such a standpoint that it enables a coating solution to be prepared for forming a film and that it has an electric charge-retaining property though it is not as high as that of the FEP (see Japanese Laid-Open Patent Publication No. 130128/1980).
However, the electret film such as of polystyrene obtained by being applied onto the electrically conducting sleeve followed by drying develops cracks when a predetermined period of time passes after drying, causing the surface potential to be attenuated, and is still unsatisfactory from the standpoint of stably conveying the electrically charged toner for extended periods of time.